1. Field of the Invention
This invention is concerned with a novel surfactant-containing fluid and an oil recovery method employing the fluid and more particularly is concerned with a novel surfactant fluid and oil recovery method employing the fluid wherein the fluid contains a mixture of petroleum sulfonates having specified even equivalent weight distribution.
2. Description of the Prior Art
It is well known to persons skilled in the art of recovery of petroleum from subterranean petroleum containing formations that only from about 10 to about 50 percent of the oil originally present in a subterranean formation can be recovered by conventional primary recovery means and by waterflooding. Substantial additional oil may be recovered from the formation by injecting into the formation an aqueous surfactant-containing aqueous fluid for the purpose of reducing the interfacial tension between water and oil, thereby increasing the microscopic displacement efficiency of the aqueous displacing fluid over that obtainable with water or field brines alone. Petroleum sulfonate is a well known and commonly utilized surfactant for surfactant flooding enhanced oil recovery processes. Petroleum sulfonate may be employed in petroleum reservoirs containing water having relatively low salinity and divalent ion concentration, e.g. salinities less than from about 5000 to about 30,000 parts per million total dissolved solids and divalent ion concentrations less than about 500 parts per million, or in formations containing water having somewhat higher salinities and divalent ion concentrations if the formation is first preconditioned by injecting into the formation in advance of the surfactant fluid, a relatively fresh water preflush to displace the higher salinity and high hardness water from the flow channels of the formation prior to injecting the petroleum sulfonate-containing fluids.
The prior art also teaches the employment of various solubilizing co-surfactants in combination with petroleum sulfonates and other organic sulfonates in order to increase the usefulness of the organic sulfonates in formations containing higher salinities and divalent ion concentrations than those referred to above. For example, U.S. Pat. Nos. 3,792,731 and 3,811,505 deal with the use of mixtures of nonionic surfactants with petroleum sulfonates and other organic sulfonates for oil recovery in formations containing high hardness water. U.S. Pat. Nos. 3,811,504; 3,811,507; 3,508,612; 3,827,497; and 3,890,239 all deal with various surfactant combinations in which petroleum sulfonate or other organic sulfonates are combined with more complex synthetic surfactants which render the mixture more soluble in solutions of higher salinities and higher concentrations of divalent ions than could be tolerated by organic sulfonates including petroleum sulfonates alone.
While the foregoing processes are effective for surfactant flooding in formations, the cost of a surfactant fluid employing a synthetic surfactant as a solubilizing co-surfactant, is generally excessive in relationship to the quantity of additional oil to be recovered.
The prior art also recognizes that certain relationships exist between the equivalent weight of petroleum sulfonates employed and their effectiveness in certain formations. For example, U.S. Pat. Nos. 3,434,542 and 3,468,377 teach the use of petroleum sulfonates for oil recovery, the petroleum sulfonate being comprised of a mixture of petroleum sulfonates having different equivalent weights, specifying the range and the maximum percentage of species whose equivalent weights are less than 290 or greater than 590.
Despite the substantial teachings discussed above, the amount of enhanced oil recovery obtainable with petroleum sulfonates has usually been unsatisfactory, particularly from a commercial point of view, and the degree of recovery obtainable from petroleum sulfonates has not always been predictable with accuracy based on the gross equivalent weight characterizations contained in the prior art references. Accordingly, there is a substantial, unfulfilled commercial need for a method for accurate characterization of the petroleum sulfonates which will most effectively and efficiently recover oil from subterranean, oil-containing formations.